Frank -- Thanks for joining us here on the forum. I just posted a "Welcome" message to you -- please be sure to take a look at that.
As for your question, the first and primary source that comes to mind on this topic is the statement from Torquatus as recorded in On Ends Book One section XVII -- see item (3) below
De finibus bonorum et malorum : Cicero, Marcus Tullius : Free Download, Borrow, and Streaming : Internet Archive
Latin and English on opposite pages
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